1. Field of the Invention
This invention relates, in general, to high temperature materials and, in particular, to a new class of aromatic phthalonitrile monomers containing ether and imide linkages and their conversion to high temperature thermosetting polymers and copolymers and the synthesis thereof.
2. Description of the Prior Art
Interest in fiber-reinforced composites for advanced aerospace applications has led to the search for high temperature polymers that are easily processed and exhibit high thermal and oxidative stability. Presently, epoxies and polyimides are used. These materials have superior mechanical properties and are lighter and more economical to produce than metals but lack the thermal stability to operate at high temperatures and tend to oxidize and become brittle over time. Conventional epoxy-based composites and adhesives are limited to 120.degree. C. maximum, have a problem with water absorption and require low temperature prepreg storage. Polyimides can produce gaseous products when cured, resulting in voids and blisters in composite components.
Phthalonitrile polymers constitute a recent and important class of high-temperature materials, having a wide range of uses, such as composite matrices, adhesives, sealants, and even semiconductors. These polymers are prepared from phthalonitriles in which the linking group between the two ortho dinitrile groups separates the dinitrile groups enough to permit polymerization. Presently several bridging groups are known. Examples include aliphatic and unsaturated groups, aromatic groups, aliphatic and aromatic diamide groups, and aliphatic and aromatic ether, sulfone and ketone groups.
The chemical and physical properties of the polymers depend primarily on the bridging groups. The groups providing the best properties are those with aromatic, polar and flexible moieties, especially the --O--.phi.--.phi.--O group of U.S. Pat. No. 4,259,471 by Keller et al, the --O--.phi.--C.sub.3 F.sub.6 --.phi.--O-- of U.S. Pat. No. 4,238,601 by Keller et al, the --O--.phi.--C.sub.3 --H.sub.6 --.phi.--O-- group of U.S. Pat. 4,223,123 by Keller et al, the --O--.phi.--SO.sub.2 --.phi.--O-- and --O--.phi.--(C.dbd.O)--.phi.--O-- groups of U.S. Pat. No. 4,234,712 by Keller el at and the --O--C.sub.n --H.sub.2n --O-- group of U.S. Pat. No. 4,226,801 by Keller el al. These polymers have exceptional thermal and oxidative stability, low water absorptivity, high strength, good dimensional integrity and strong adhesion. The aromatic moieties provide the high mechanical strength, modulus and high thermal and oxidative stability and the polar moieties provide the excellent adhesive properties.
U.S. Pat. No. 4,408,035 teaches curing of phthalonitrile monomers with a nucleophilic aromatic amine. The monomer, 4,4'-bis(3,4-dicyanophenoxy)biphenyl, has a melting point of 232.degree.-234.degree. C. The aromatic diamines covered in the above patent are somewhat volatile at the required processing melt temperature, causing void problems when used in an amount greater than 5% by weight. It is advantageous for a resin not to produce gaseous products when cured. Also, the chemical makeup of the polymer must be such that it consists of units having known resistance to bond-rupture under thermal, oxidative and hydrolytic conditions.
U.S. patent application Ser. No. 07/273,443, U.S. Pat. No. 5,003,039 discloses 1,3bis(3-aminophenoxy)benzene and other bis(aminophenoxy) compounds used as a curing agent for a rapid synthesis of phthalonitrile resin. The time and temperature needed for polymerization of bisphenol-linked phthalonitrile monomers are easily controlled as a function of the concentration of amine curing agent.
The necessity for aromatic and heterocyclic ring structure in a polymer to achieve heat resistance has long been recognized. The ideal heat resistant polymer would be composed of aromatic and/or heteroaromatic ring structures interconnected by flexible linkages within the polymeric backbone to improve processability and to enhance the mechanical properties. However, few synthetic methods are available for incorporating stable linkages into a polymeric system.